Exclude reflection_tester from release (#8892)

Protobuf-C++.podspec

diff --git a/Protobuf-C++.podspec b/Protobuf-C++.podspec
index f54f548..3ebb5cc 100644
--- a/Protobuf-C++.podspec
+++ b/Protobuf-C++.podspec
@@ -22,6 +22,7 @@
                     'src/google/protobuf/test_util*.{h,cc}',
                     'src/google/protobuf/map_lite_test_util.{h,cc}',
                     'src/google/protobuf/map_test_util*.{h,cc,inc}'
+                    'src/google/protobuf/reflection_tester.{h,cc}'
 
   s.header_mappings_dir = 'src'